Why does my procedural fill not show all the tiles I set?

GameMaker batches tilemap renders by chunk. Setting many cells in a tight loop can race the cache. Call tilemap_clear first if you're rebuilding from scratch; otherwise sets should propagate but visualize one frame later.

Should I tilemap_clear before fill?

For wholesale rebuilds, yes — clear then set. For incremental edits, set directly. Repeated full clears + fills are wasteful; batch into a single rebuild pass.

How do I detect tile data corruption?

Read back via tilemap_get(tm, x, y) immediately after setting. If the read doesn't match what you wrote, the layer index or coordinate system is off. Verify tilemap layer ID.

Fix: GameMaker Tilemap Cache Stale After Fill

Quick answer: Call tilemap_clear(tm, 0) before a wholesale rebuild, then loop sets. Verify with tilemap_get immediately after to detect coordinate or layer ID errors.

Procedural map gen sets thousands of tiles. Some don’t appear. Cache + render order can race; clearing first is safe.

The Symptom

tilemap_set_cell calls return success but visual shows missing/wrong tiles. Restarting the room sometimes fixes; sometimes not.

The Fix

/// Procedural rebuild
tilemap_clear(tm, 0);

for (var y = 0; y < 100; ++y) {
    for (var x = 0; x < 100; ++x) {
        var tile = choose_tile_for(x, y);
        tilemap_set(tm, tile, x, y);
    }
}

Clear, then loop. Cache rebuilds cleanly because every cell is freshly written.

Verifying

Read back random cells immediately after set: tile = tilemap_get(tm, x, y). Should match what you wrote. Mismatch points to layer ID or coord system error.

“Clear before mass set. Verify reads match writes. Cache stays current.”

Related Issues

For network buffer fragmentation, see fragmentation. For surface lost, see surface lost.

Clear plus rebuild. Tiles render.